Apparatus for capturing and removing contaminant particles from an interior region of an ion implanter

ABSTRACT

A method of capturing and removing contaminant particles moving within an evacuated interior region of an ion beam implanter is disclosed. The steps of the method include: providing a particle collector having a surface to which contaminant particles readily adhere; securing the particle collector to the implanter such that particle adhering surface is in fluid communication to the contaminant particles moving within the interior region; and removing the particle collector from the implanter after a predetermined period of time. An ion implanter in combination with a particle collector for trapping and removing contaminant particles moving in an evacuated interior region of the implanter traversed by an ion beam is also disclosed, the particle collector including a surface to which the contaminant particles readily adhere and securement means for releasably securing the particle collector to the implanter such that the particle adhering surface is in fluid communication with the evacuated interior region of the implanter.

FIELD OF INVENTION

The present invention concerns a method and an apparatus for capturingand removing contaminant particles moving within an interior region ofan ion implanter and, more particularly, capturing contaminant particlesby securing a particle collector having a contaminant particle adheringsurface in fluid communication with the interior region of theimplanter.

BACKGROUND OF THE INVENTION

Ion implanters are used to implant or "dope" silicon wafers withimpurities to produce n or p type extrinsic materials. The n and p typeextrinsic materials are utilized in the production of semiconductorintegrated circuits. As its name implies, the ion implanter dopes thesilicon wafers with a selected ion species to produce the desiredextrinsic material. Implanting ions generated from source materials suchas antimony, arsenic or phosphorus results in n type extrinsic materialwafers. If p type extrinsic material wafers are desired, ions generatedwith source materials such as boron, gallium or indium will beimplanted.

The ion implanter includes an ion source for generating positivelycharged ions from ionizable source materials. The generated ions areformed into a beam and accelerated along a predetermined beam path to animplantation station. The ion implanter includes beam forming andshaping structure extending between the ion source and the implantationstation. The beam forming and shaping structure maintains the ion beamand bounds an elongated interior cavity or region through which the beampasses en route to the implantation station. When operating theimplanter, the interior region must be evacuated to reduce theprobability of ions being deflected from the predetermined beam path asa result of collisions with air molecules.

For high current ion implanters, the wafers at the implantation stationare mounted on a surface of a rotating support. As the support rotates,the wafers pass through the ion beam. Ions traveling along the beam pathcollide with and are implanted in the rotating wafers. A robotic armwithdraws wafers to be treated from a wafer cassette and positions thewafers on the wafer support surface. After treatment, the robotic armremoves the wafers from the wafer support surface and redeposits thetreated wafers in the wafer cassette.

Operation of an ion implanter results in the production of certaincontaminant particles. One source of contaminant particles isundesirable species of ions generated in the ion source. Contaminantparticles with respect to a given implant result from the presence ofresidual ions from a previous implant in which different ions wereimplanted. For example, after implanting boron ions in a given number ofwafers, it may be desired to change over the implanter to implantarsenic ions. It is likely that some residual boron atoms remain in theinterior region of the implanter.

Yet another source of contaminant particles is photoresist material.Photoresist material is coated on wafer surfaces prior to implantationand is required to define circuitry on the completed integrated circuit.As ions strike the wafer surface, particles of photoresist coating aredislodged from the wafer.

Contaminant particles which collide with and adhere to wafers during iontreatment are a major source of yield loss in the fabrication ofsemiconductor and other devices which require submicroscopic patterndefinition on the treated wafers.

In addition to rendering the implanted or treated wafers unsuitable fortheir intended purpose in the manufacture of integrated circuits,contaminant particles adhering to interior surfaces of the ion implanterreduce the efficiency of ion implanter components, for example, theperformance of an ion beam neutralization apparatus will bedetrimentally effected by a build-up of photoresist particles on theapparatus' aluminum extension tube.

The vacuum environment of the implanter interior makes capture andremoval of contaminant particles problematical. In a vacuum, the motionof submicroscopic particles is extremely difficult to control, particlemovement is controlled to a great extent by electrostatic forces.Gravitational forces become insignificant with decreasing particle size.

It has been found that particles moving within the evacuated interior ofthe implanter bounce or rebound numerous times before settling on andadhering to the workpiece surface or to an interior surface of theimplanter. Experience indicates that such a moving particle may bounce10 to 25 times before settling.

In essence, a particle collector includes a particle adhering surface.Particles colliding with the surface become attached thereto and areremoved when the collector is removed. However, for a particle collectorto be used in conjunction with an ion implanter, the particle collectorwould have to be compatible with the vacuum environment. Conventionalparticle collector surfaces, e.g., adhesives, porous materials, oilymaterials, etc. tend to outgas in a vacuum environment which makes theminappropriate for use in the implanter. What is needed is a particlecollector for contaminant particles which is suitable for use in avacuum environment and which exhibits high particle adhering qualities.

DISCLOSURE OF THE INVENTION

The present invention provides a method and an apparatus for capturingand removing contaminant particles that move through and land oninterior surfaces of an ion implanter. One or more particle collectorshave a particle adhering surface positioned within an interior region ofan ion implanter. Contaminants that bounce multiple times off theinterior walls have a high probability of being captured by the one ormore particle adhering surfaces of the particle collectors.

Specifically, the method of the present invention of capturing andremoving contaminant particles moving within an interior region of anion beam implanter includes the steps of: providing a particle collectorhaving a surface to which contaminant particles readily adhere to;securing the particle collector to the implanter such that the particleadhering surface of the collector is in fluid communication with theimplanter interior region; and removing the particle collector from theimplanter after a predetermined period of time.

An ion implanter in combination with a particle collector for trappingand removing contaminant particles moving in an evacuated interiorregion of the implanter traversed by an ion beam is also disclosed. Theparticle collector includes a surface to which the contaminant particlesreadily adhere and securement means for securing the particle collectorto the implanter such that the particle adhering surface is within aclear field of view of regions that tend to generate contaminants withinthe implanter.

One or more particle collectors may be advantageously positioned atvarious locations including inside the resolving housing and inside theprocess chamber. The implanter includes an ion beam resolving housingdefining a portion of the interior region traversed by the ion beam. Theresolving housing interior region is evacuated. The particle collectormay be positioned such that the particle adhering surface is in fluidcommunication with the portion of the interior region defined by theresolving housing.

The implanter also includes a wafer implantation process chamberdefining a portion of the interior region. The particle collector may bepositioned such that the particle adhering surface is in fluidcommunication with the portion of the interior region defined by theprocess chamber.

The particle adhering surface may additionally attract the contaminantparticles. As an example, electret fibers can be used to attract andsecure the particles to the particle collector by electrostaticattraction. Alternatively, the particle adhering surface may be coatedwith partially cured elastomers which secure the particles to theparticle collector by surface tension. Silicone elastomer is a preferredelastomer.

These and other objects, advantages and features of the invention willbecome better understood from a detailed description of a preferredembodiment of the invention which is described in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view, partly in section, showing an ion implanterincluding an ion source, beam forming and shaping structure and animplantation chamber;

FIG. 2 is an enlarged plan view of an electron shower portion of the ionimplanter of FIG. 1;

FIG. 3 is a schematic depiction of portions of an ion beam implantershowing portions of the implanter particularly suited for placement ofone or more particle collectors for trapping contaminants; and

FIG. 4 is a perspective view of an ion neutralization tube that isconfigured as a particle trap.

DETAILED DESCRIPTION

Turning now to the drawings, FIG. 1 depicts an ion implanter, showngenerally at 10, which includes an ion source 12 for emitting ions thatform an ion beam 14 and an implantation station 16. Control electronics11 are provided for monitoring and controlling the ion dosage receivedby the wafers within a process chamber 17 at the implantation station16. The ion beam 14 traverses the distance between the ion source 12 andthe implantation station 16.

The ion source 12 includes a plasma chamber 18 defining an interiorregion into which source materials are injected. The source materialsmay include an ionizable gas or vaporized source material. Sourcematerial in solid form is deposited into a pair of vaporizers 19. Thevaporized source material is then injected into the plasma chamber.

Energy is applied to the source materials to generate positively chargedions in the plasma chamber 18. The positively charged ions exit theplasma chamber interior through an elliptical arc slit in a cover plate20 overlying an open side of the plasma chamber 18.

An ion source utilizing microwave energy to ionize source materials isdisclosed in U.S. patent application Ser. No. 08/312,142, now U.S. Pat.No. 5,523,652 filed Sep. 26, 1994, which is assigned to the assignee ofthe instant application. U.S. patent application Ser. No. 08/312,142 isincorporated herein in its entirety by reference. The ion beam 14travels through an evacuated path from the ion source 12 to theimplantation station 17, which is also evacuated. Evacuation of the beampath is provided by vacuum pumps 21.

Ions in the plasma chamber 18 are extracted through the arc slit in theplasma chamber cover plate 20 and accelerated toward a mass analyzingmagnet 22 by a set of electrodes 24 adjacent the plasma chamber coverplate 20. Ions that make up the ion beam 14 move from the ion source 12into a magnetic field set up by the mass analyzing magnet 22. The massanalyzing magnet is part of the ion beam forming and shaping structure13 and is supported within a magnet housing 32. The strength of themagnetic field is controlled by the control electronics 11. The magnet'sfield is controlled by adjusting a current through the magnet's fieldwindings. The mass analyzing magnet 22 causes the ions traveling alongthe ion beam 14 to move in a curved trajectory. Only those ions havingan appropriate atomic mass reach the ion implantation station 16.

Along the ion beam travel path from the mass analyzing magnet 22 to theimplantation station 16, the ion beam 14 is further shaped, evaluatedand accelerated due to the potential drop from the high voltage of themass analyzing magnet housing 32 to the grounded implantation chamber.

The ion beam forming and shaping structure 13 further includes aquadrupole assembly 40, a moveable Faraday cup 42 and an ion beamneutralization apparatus 44. The quadrupole assembly 40 includes set ofmagnets 46 oriented around the ion beam 14 which are selectivelyenergized by the control electronics (not shown) to adjust the height ofthe ion beam 14. The quadrupole assembly 40 is supported within ahousing 50.

Coupled to an end of the quadrupole assembly 40 facing the Faraday flag42 is an ion beam resolving plate 52. The resolving plate 52 iscomprised of vitreous graphite and is shown in FIG. 3. The resolvingplate 52 includes an elongated aperture 56 through which the ions in theion beam 14 pass as they exit the quadrupole assembly 40. The resolvingplate 52 also includes four counterbored holes 58. Screws (not shown)fasten the resolving plate 52 to the quadrupole assembly 40. At theresolving plate 52 the ion beam dispersion, as defined by the width ofthe envelope D', D", is at its minimum value, that is, the width of D',D" is at a minimum where the ion beam 14 passes through the resolvingplate aperture 56.

The resolving plate 52 functions in conjunction with the mass analyzingmagnet 22 to eliminate undesirable ion species from the ion beam 14. Thequadrupole assembly 40 is supported by a support bracket 60 and asupport plate 62. The support bracket 60 is coupled to an interiorsurface of the resolving housing 50 while the support plate 62 iscoupled to an end of the housing 50 via a plurality of screws (twoscrews 63 fastening the support plate 62 to the housing 50 is seen inFIG. 2). Attached to the support plate 62 is a quadrupole assemblyshield plate 64 (shown in FIG. 4). The quadrupole assembly shield plate64 is comprised of vitreous graphite and includes a rectangular aperture66 and four counterbored holes 68. The counterbored holes 68 acceptscrews which secure the quadrupole assembly shield plate 64 to thesupport plate 62 (two screws 70 extending through two of thecounterbored holes 68 and into the support plate 62 is seen in FIG. 2).The quadrupole assembly shield plate 64 protects the quadrupole assembly40 from impact by undesirable ions having an atomic mass that is "close"to the atomic mass of the desired ion species. During operation of theimplanter 10, undesirable ions impacting an upstream facing surface ofthe quadrupole assembly shield plate 64 build-up the plate.

As can be seen in FIG. 1, the Faraday flag 42 is located between thequadrupole assembly 40 and the ion beam neutralization apparatus 44. TheFaraday flag is moveable relative to the housing 50 so that it can beslid into position to intersect the ion beam 14 to measure beamcharacteristics and, when the measurements are satisfactory, moved outof the beam line so as to not interfere with wafer implantation at theimplantation chamber 17.

The beam forming structure 13 also includes the ion beam neutralizationapparatus 44, commonly referred to as an electron shower. U.S. Pat. No.5,164,599 to Benveniste, issued Nov. 17, 1992, discloses an electronshower apparatus in an ion beam implanter and is incorporated herein inits entirety by reference. The ions extracted from the plasma chamber 18are positively charged. If the positive charge on the ions is notneutralized at the wafer's surface, the doped wafers will exhibit a netpositive charge. As described in the '599 patent, such a net positivecharge on a wafer has undesirable characteristics.

The ion beam neutralization apparatus 44 shown in FIG. 2 includes a biasaperture 70, a target 72 and an extension tube 74. Each of the biasaperture 70, the target 72 and the extension tube 74 are hollow and whenassembled define an open ended, cylindrical interior region throughwhich the ion beam 14 passes and is neutralized by secondary electronemissions. The neutralizer apparatus 44 is positioned with respect tothe housing 50 by a mounting flange 76 which bolts to the resolvinghousing.

Extending from the mounting flange 76 is a support member 78 and thebias aperture 70. The target is secured to the support member 78. Theextension tube 74 is coupled to, but electrically isolated from, thetarget 72. The extension tube 74 is grounded by a connection with agrounding terminal G. The bias aperture 70 is energized with a negativecharge V-. The support member 78 defines an interior passageway (notshown) for the circulation of cooling fluid.

The support member 78 also supports a filament feed 80 electricallycoupled to a set of filaments (not shown). The filaments extend into thetarget 72 and, when energized, emit high energy electrons which areaccelerated into an interior region of the target 72. The high energyelectrons impact the interior wall of the target 72. The collisions ofthe high energy electrons with the target interior wall causes anemission of low energy electrons or so-called secondary electronemission.

Particle Traps

A preferred extension tube 74 of the beam neutralizer is constructedfrom a contaminant capturing material and hence forms a particle trap.More particularly the tube 74 is constructed from a machined cylinder ofaluminum foam. A solid cylindrical molded piece is then extracted fromthe mold and machined to define a throughpassage P having a diameter toaccommodate the dimensions of the ion beam that passes through theextension tube 74.

Stray contaminants can become entrapped within the ion beam 14 andcarried along with the beam into the beam neutralizer 44. Use of thetube 74 tends to collect any such contaminating particles that arewithin the beam and which have travel paths along the beam border. Theycan collide with the tube 74 and are not only removed from the beam andhence do not bombard the target within the implantation chamber, butthey do not bounce off the surface of the tube 74 due to the makeup ofthe tube.

The tube 74 is constructed from the foam aluminum material and has ahigh surface area. An inner surface that bounds the throughpassage Ptends to reduce bouncing of stray contamination particles off the wallsof the throughpassage. The tube's inner surface has many irregularities,pockets, depressions and crevices. The aluminum forms a latticestructure of connected segments interspaced by irregularly organizedvoids into which the stray contaminants can center and become trapped.

The preferred aluminum foam is sold under the designation Duocel(Registered Trademark) by Energy Research and Generation, Inc. ofOakland, Calif. The material has been used in the prior art as aconstruction material where high mechanical strength is required butwhere lightweight construction materials are necessary.

Implantation Chamber 17

The implantation station 16 includes the evacuated implantation chamber17 (FIGS. 1 and 3). Rotatably supported within the implantation chamber17 is a disk shaped wafer support (not shown). Wafers to be treated arepositioned near a peripheral edge of the wafer support and the supportis rotated by a motor (not shown) at about 1200 RPM. The ion Beam 14impinges on the wafers and treats the wafers as they rotate in acircular path.

The implantation station 16 is pivotable with respect to the housing 50and is connected thereto by a flexible bellows 92. The ability to pivotthe implantation station 16 permits adjustments to the angle ofincidence of the ion beam 14 on the wafer implantation surface.

FIG. 3 schematically indicates components of the ion implanter 10 towhich material constructed from the foam aluminum could conveniently beattached. These other particle collectors could supplement or be used inplace of the tube 74 shown in FIG. 4. Instead of supported by theexisting beam neutralizer apparatus, such additional collectors havingsurfaces S1-S6 would be constructed from generally planar sheets ofaluminum foam that have a thickness of 0.25 inches and whose outerdimensions vary depending on their intended position within theimplanter.

Tradeoffs are involved in the choice of material density for use in theparticle trap. More accurately, the pore density of the aluminum foamhas been analyzed to determine optimum material characteristics. Thealuminum foam material is available in a variety of pore densitiesranging from about 10 pores per inch to about 40 pores per inch. (Note,the description of foam porosity in units of pores per inch originateswith Energy Research and Generation, Inc.) In choosing the correctmaterials two principle issues are considered.

A first issue is particle trap efficiency. A suitable thickness for thefoam is controlled to a degree by mechanical constraints such as theclearance needed between the rotating disk that supports the wafers andthe process chamber walls. Once the preferred thickness of 0.25 inch isspecified, the foams porosity must be chosen.

The porosity must be chosen so that particles have a negligibleprobability of bouncing off from the foam. The foam needs to be porousenough to allow use of most of the interior surface area of the foammaterial to assure the particle trapping efficiency is as high aspossible. A foam which is too porous will not work because particleswould simply pass through it, bounce off the walls of the ion implanterand again pass through the foam. A foam with too little porosity wouldreflect many particles from its front surface.

A second issue is controlled by the mechanical strength and ease infabrication of the foam. In general coarser foams (10 pores per inch)are harder to fabricate, especially in the geometry of particle trapsused with ion implanters.

These two issues cause a pore density of about 20 pores per inch to bechosen for the 0.25 inch thick foam. This results in a foam density of 6to 8 percent of the bulk aluminum by weight. Another criterion for boththe sheets and the tube 74 is that it must be able to be periodicallyreplaced so that as contaminants build up they can be removed from theion implanter. The tube 74 is attached to the beam neutralizer 44 bymeans of connectors C that extend through four equally spaced mountingtabs T integrally formed with the target 72.

The process chamber 17 of FIG. 3 is seen to include two vacuum ports V3,V4 for controllably pressurizing and depressurizing the process chamber17 when maintenance is performed on the implanter. Wafers are insertedinto the chamber 17 by means of a load lock 110 and mounted onto arotating support that carries the wafers through the ion beam.Photoresist on the wafers tends to be ejected off from the wafers andcan collect on the interior surfaces S1, S2, S3, S4 of collector sheetsmounted within the process chamber.

Mounting of the sheets within the process chamber or along surfaces S5,S6 within the resolving housing 112 is accomplished with a specialadhesive. In accordance with the invention, a room temperature curedvulcanized silicone adhesive (RTV) is applied to the interior surface ofthe ion implanter and the aluminum foam sheet is applied to the treatedsurface. This process has been found to adequately secure the sheetwithin the implanter without producing further contaminants inside theprocess chamber 17 or the resolving housing 112.

While the present invention has been described in some degree ofparticularity, it is to be understood that those of ordinary skill inthe art may make certain additions or modifications to, or deletionsfrom, the described present embodiment of the invention withoutdeparting from the spirit or scope of the invention, as set forth in theappended claims.

We claim:
 1. In combination, an ion implanter and apparatus for trappingand removing contaminant particles moving in an evacuated interiorregion of the ion implanter traversed by ions moving toward a workpiece,the apparatus comprising:a) a collector having a particle collectingsurface to which contaminant particles moving through the evacuatedinterior region of the ion implanter readily adhere; and b) a supportfor removably mounting the collector within the evacuated interiorregion of the ion implanter to position the particle collecting surfaceof the collector for intercepting contaminant particles moving throughthe evacuated interior region of the ion implanter.
 2. The combinationof claim 1, wherein the ion implanter comprises a workpiece supportchamber,wherein the particle collecting surface of the collector isgenerally planar, and wherein the collector is attached to an interiorsurface of the workpiece support chamber.
 3. The combination of claim 1,wherein the ion implanter comprises a housing that defines a portion ofthe evacuated interior region of the ion implanter,wherein the ionimplanter comprises a source for emitting ions and beam formingstructure for creating an ion beam for beam treatment of one or moreworkpieces, and wherein the collector has an annular shape and thesupport comprises means for supporting a cylindrical inner surface ofthe collector in a position such that an ion beam passes through thecollector on its way to a workpiece.
 4. The combination of claim 3,wherein the ion implanter comprises an ion implantation chamberincluding a workpiece support for moving one or more workpieces throughthe ion beam andwherein one or more additional collectors are attachedto an interior surface of the ion implantation chamber.
 5. Thecombination of claim 1, wherein the particle collecting surface of thecollector attracts the contaminant particles.
 6. The combination ofclaim 5, wherein the particle collecting surface of the collectorincludes electret fibers which attract and secure the contaminantparticles to the particle collecting surface by electrostaticattraction.
 7. The combination of claim 1, wherein the particlecollecting surface of the collector includes a coating of partiallycured elastomers which secure the contaminant particles to the particlecollecting surface by surface tension.
 8. The combination of claim 7,wherein the coating of partially cured elastomers includes a siliconeelastomer.
 9. The combination of claim 1, wherein the collectorcomprises a metal foam having a controlled thickness.
 10. Thecombination of claim 9, wherein the metal foam is constructed fromaluminum.
 11. The combination of claim 9, wherein the metal foam haspores and a controlled density based on the density of the pores. 12.The combination of claim 9, wherein the metal foam has a foam density of6 to 8 percent of metal for the metal foam by weight.